1. Field of the Invention
The present invention relates to a process for producing 7-octen-1-al by isomerizing 2,7-octadien-1-ol.
2. Discussion of the Background
The compound 7-octen-1-al contains a terminal double bond and an aldehyde group with high reactivity and is very useful as a starting material for various industrial chemicals.
For instance, 7-octen-1-al can be converted into 1,9-nonanedial by hydroformylation, which can give various starting materials for synthesizing polymers, such as 1,9-nonanediol, azelaic acid and 1,9-nonanediamine. Also, 7-octen-1-al can be converted to 7-octenoic acid by oxidation with oxygen in the presence of a catalyst such as a cobalt salt, a manganese salt, a nickel salt, a copper salt or an iron salt. Furthermore, 7-octen-1-al can be converted by reduction into 7-octen-1-al or 1-octanol, or can be converted to capric acid by reduction of the double bond and oxidation of the aldehyde group.
Japanese Patent Publication No. 60378/1987 and Japanese Patent Application Laid-Open No. 118535/1983 disclose a process for producing 7-octen-1-al, which comprises isomerizing 2,7-octadien-1-ol in the presence of a catalyst selected from the group consisting of copper catalysts and chromium catalysts. Japanese Patent Application Laid-Open No. 218638/1990 (Patent No. 2565561) discloses a process for producing 7-octen-1-al, which comprises isomerizing, at a temperature of 180-250.degree. C., 2,7-octadien-1-ol in the presence of at least one compound selected from the group consisting of n-octanol, 3-octanol and 7-octen-1-ol in an amount of 50-200% by weight based on the weight of 2,7-octadien-1-ol using a metal oxide catalyst containing at least two metals selected from the group consisting of copper, chromium and zinc.
In the process described in Japanese Patent Publication No. 60378/1987 and Japanese Patent Application Laid-Open No. 118535/1983, there is inevitably produced as a byproduct, 2,7-octadien-1-al which it is difficult to separate by distillation from the product 7-octen-1-al. The byproduct 2,7-octadien-1-al, is a catalyst poison for hydroformylation and decreases the yield of 1,9-nonanedial on hydroformylation of the obtained 7-octen-1-al. This fact is clear from the description in Japanese Patent Application Laid-Open No. 218638/1990 that when the reaction is conducted in accordance with the procedure given in the Examples of Japanese Patent Publication No. 60378/1987 with repeated use of the same catalyst, there occurs a gradual increase of 2,7-octadien-1-al. The process described in Japanese Patent Application Laid-Open No. 218638/1990 decreases the amount of 2,7-octadien-1-al byproduct to a low level. Example 2 shows a level of only 0.7%, which is the largest figure in the Examples after a 7-hour reaction. However, with this process, n-octanol and 3-octanol, which are used as sources for generating hydrogen, are necessarily converted by dehydrogenation during the reaction, into n-octylaldehyde (boiling point: 173.degree. C./760 mmHg) and 3-octanone (boiling point; 168.degree. C./760 mmHg) respectively, each having a boiling point very close to that (174.degree. C./760 mmHg) of 7-octen-l al. As a result, the process described in Japanese Patent Application Laid-Open No. 218638/1990 is deficient since it is very difficult to separate the desired product from these byproducts by distillation.
In order to solve the above problem of the process described in Japanese Patent Application Laid-Open No. 218638/1990, it is necessary to develop a process which does not use n-octanol, 3-octanol or 7-octen-1-ol. The present inventors have, therefore, studied in detail the mechanism of formation of 7-octen-1-al from 2,7-octadien-1-ol. They discovered that the reaction is a formal isomerization which comprises dehydrogenation of the allyl alcohol part of 2,7-octadien-1-ol to form 2,7-octadien-1-al and the succeeding hydrogenation of the carbon-carbon double bond conjugating with the aldehyde group to form 7-octen-1-al. Thus, the production of 2,7-octadien-1-al in this reaction is due to the fact that hydrogen formed by dehydrogenation is consumed not only for hydrogenation to form 7-octen-1-al, but also for the formation of various byproducts, thereby causing a part of the intermediate product 2,7-octadien-1-al to remain in the reaction zone. Indeed, the present inventors conducted isomerization in accordance with the procedures given in Comparative Examples 1, 3, 4 and 5 in the instant specification to confirm that 2,7-octadien-1-al was produced in large amounts. The present inventors also discovered that isomerization over a long period of time, as shown in Comparative Example 6 of the instant specification resulted in an increase in the amount of 2,7-octadien-1-al product and, at the same time, a decrease in the conversion of 2,7-octadiene-1-ol, so that the selectivity to 7-octen-1-al decreases. In summary, a serious problem was found in that 2,7-octadien-1-al decreased the catalytic activity for the isomerization.
Furthermore, when the reaction is conducted by a reaction-distillation method, which is a particularly referred embodiment in Japanese Patent Publication No. 60378/1987, Japanese Patent Application Laid-Open No. 118535/1983 and Japanese Patent Application Laid-Open No. 218638/1990, another problem occurs since high-boiling byproducts inevitably result originating from the aldehydes formed by the reaction, and this leads to a decrease in the yield.
It is, therefore, important, if the reaction is to be conducted over a period of time, to suppress the formation of 2,7-octadien-1-al (which decreases the catalytic activity) and the production of high-boiling byproducts. As described above, the mechanism involved in the formation of 7-octen-1-al from 2,7-octadien-1-ol is a formal isomerization which involves dehydrogenation of the allyl alcohol part of 2,7-octadien-1-ol to form 2,7-octadien-1-al and the hydrogenation of the carbon-carbon double bond conjugating with the aldehyde group to form 7-octadien-1-al. The byproduction of 2,7-octadien-1-al is a result of the fact that the hydrogen formed on dehydrogenation is consumed not only for hydrogenation to 7-octen-1-al, but for various side reactions. This leads to insufficient hydrogenation of 2,7-octadion-1-al and thus allows this aldehyde to remain in the reaction zone. Since hydrogenation of the remaining 2,7-octadien-1-al by adding hydrogen (which is inexpensive) to the reaction zone would reduce the content of 2,7-octadien-1-al in the final reaction mixture, the inventors decided to study the hydrogenation conditions in detail. The specification of Japanese Patent Publication No. 60378/1987 states: "The isomerization reaction is preferably carried out in an atmosphere of a gas which is inert under the reaction condition. The whole or part of the inert gas may be replaced with hydrogen gas. When the reaction is carried out in the co-presence of hydrogen, the hydrogen partial pressure is preferably kept below 10 atmospheres. At a hydrogen partial pressure exceeding 10 atmospheres, the hydrogenation reaction prevails, unfavorably causing decrease in the selectivity toward 7-octen-1-al." and the same description is disclosed in the specification of Japanese Patent Application Laid-Open No. 118535/1983. However, the specification of Japanese Patent Publication No. 60378/1987 only discloses in its Examples 2 and 4, that the ratios of isomerization to produce 7-octen-1-al under a hydrogen atmosphere were 43% and 73%, respectively, and gives no description of the amount of hydrogen added. The specification of Japanese Patent Application Laid-Open No. 118585/1983 discloses in its Examples 4 and 6 the results that the selectivity toward 7-octen-1-al production under the flow of hydrogen gas were 92% and 79%, respectively. The present inventors have studied this reaction and found the selectivity toward 7-octen-1-al production is low, that is, the byproduction of other hydrogenation products prevails in this reaction, and the activity of the catalyst decreases as the reaction proceeds. This is shown in Comparative Example 7 of the specification. The catalysts disclosed in these prior art citations are known catalysts for hydrogenation. It would be expected, therefore, that addition of hydrogen to the reaction zone would cause various hydrogenation side reactions to occur and, in some cases, further hydrogenation of 7-octen-1-al thereby decreasing the reaction selectivity to 7-octen-1-al. However, the prior art contains no suggestion of the effect of the addition of hydrogen to suppress byproduction of 2,7-octadien-1-al.